Magnetic recording disk drives incorporate stacked, commonly-rotated rigid magnetic recording disks that are used for storage of user data. The data is recorded in radially-spaced data tracks on the surfaces of the disks. Recording heads are moved by an actuator in a generally radial path toward and away from the center of rotation of the disks to read and write data on the disks. Typically, a single recording head, which may be an inductive read/write head or an inductive write head in combination with a magnetoresistive read head, is associated with a corresponding magnetic recording surface of each disk.
It is necessary to know the precise radial and circumferential location of the recording heads relative to their associated disk surfaces. Radial position information is recorded on the disk as servo information and is used to locate the heads to the desired data tracks and maintain the heads over the desired data tracks during read and write operations. Circumferential position information is used to identify the start of different data fields located around the tracks. In conventional fixed-block architecture disk drives, the data is located in fixed bit length angular data sectors and the servo information is located in angularly-spaced servo sectors that are interspersed among the data sectors. This type of disk drive is referred to as a sector servo or embedded servo disk drive.
In most disk drives, it is important to accurately determine the servo sector number (SSN). This is especially true for disk drives using a headerless architecture, where there are no identification (ID) fields to locate the data sectors and the data sector numbers are derived from the SSNs. In some disk drive servo systems, the servo controller uses knowledge of the SSN to compensate for effects such as repeatable runout, which is a predictable head position error caused by the disk being not precisely centered over the axis of rotation of the spindle motor. In many disk drives, it is beneficial for the drive to determine the SSN as quickly as possible following a head switch, to speed read and write operations.
Accurate and rapid determination of SSNs is also especially important in disk drives used in portable or laptop computers that use idle modes to reduce power consumption. A typical idle mode will turn off power to the read, write and servo electronics, but keep the disk spinning. This provides for a significant power saving, but suffers from a recovery delay in returning to the fully operational (active) state. In a headerless drive, data read or write operations require knowledge of the SSN. Acquiring the SSN quickly can significantly reduce the recovery delay. Improving the recovery latency will provide additional opportunities for reducing drive power consumption using one or more of these idle modes.
In the prior art, the SSN is determined using any of a number of methods. The most common methods used are encoding an index mark on the track, and counting the SSNs from the index mark; and recording the SSN within the servo sector and then reading the SSN. Variations include combinations of these techniques, where both an index mark and some portion of the SSN are encoded within the servo sectors. However, all of these methods are susceptible to errors in the servo sectors, compromising reliability. Errors in the index sector may result in the loss of index, and errors in other sectors may cause loss of the associated SSN. The index plus count method suffers from a large latency when initially acquiring the SSN, since it must make a determination of the index location. Recording the SSN within the servo sectors reduces the disk drive's data storage capacity due to the overhead of the increased number of bits in the servo sector.
What is needed is a disk drive that can accurately and rapidly identify the servo sectors without reducing the data storage capacity.